Identifying Unknown Substances | 未知物质的鉴定

📚 Identifying Unknown Substances | 未知物质的鉴定

Chemical identification is a core skill in A-Level Chemistry. Examiners expect you to draw on qualitative tests, observations, and spectroscopic data to determine the identity of an unknown substance.

化学鉴定是A-Level化学的核心技能。考官期望你运用定性测试、观察现象和波谱数据来确定未知物质的身份。

1. The Command Word ‘Identify’ | 命令词“Identify”

In AQA A-Level Chemistry, the command word “identify” means you must give the name, formula, or structure of the species. You are not asked to explain how you arrived at it, although showing your reasoning can still gain partial credit in longer questions.

在AQA A-Level化学中，命令词“identify”意味着你必须给出该物种的名称、化学式或结构。它并不要求你解释如何得出这个结论，不过在较长的题目中展示你的推理过程仍然可以获得部分分数。

For example, if a question says “Identify the gas produced when sodium carbonate reacts with dilute hydrochloric acid,” you should simply write “carbon dioxide.”

例如，如果题目说“鉴定碳酸钠与稀盐酸反应产生的气体”，你只需写“二氧化碳”。

A common mistake is to write “CO₃²⁻” when asked to identify a solid, or to describe a colour change instead of naming the substance. Read the command word carefully.

一个常见错误是在要求鉴定固体时写出“CO₃²⁻”，或者只描述颜色变化而不给出物质名称。请仔细阅读命令词。

2. Flame Tests for Metal Ions | 金属离子的焰色测试

Flame tests are used to identify certain metal cations by the characteristic colour they impart to a non-luminous Bunsen burner flame.

焰色测试用于通过某些金属阳离子在无光本生灯火焰中产生的特征颜色来鉴别它们。

Metal ion	Flame colour	中文名称
Li⁺	Crimson red	锂离子深红色
Na⁺	Yellow/orange	钠离子黄色/橙色
K⁺	Lilac	钾离子淡紫色
Ca²⁺	Brick red	钙离子砖红色
Ba²⁺	Apple green	钡离子苹果绿色
Cu²⁺	Blue-green	铜离子蓝绿色

Before the test, the platinum wire is cleaned by dipping it in concentrated hydrochloric acid and heating it in the flame until no colour remains. This removes contaminating ions.

测试前，铂丝需用浓盐酸浸洗并在火焰中加热至无色，以去除杂质离子。

Flame tests are useful for identifying sodium, potassium, calcium, barium, and copper ions, but they cannot detect all ions. Lithium gives a crimson flame; strontium also gives red, so a flame test alone may not be definitive.

焰色测试对鉴定钠、钾、钙、钡和铜离子很有用，但不能检测所有离子。锂产生深红色火焰，锶也产生红色，因此仅凭焰色测试可能不够确定。

3. Testing Cations with Sodium Hydroxide | 用氢氧化钠测试阳离子

Adding aqueous sodium hydroxide to a solution containing a metal cation can produce a characteristic precipitate. The colour and solubility of the precipitate help identify the cation.

向含有金属阳离子的溶液中加入氢氧化钠水溶液会产生特征沉淀。沉淀的颜色和溶解性有助于鉴定阳离子。

Cu²⁺ gives a blue precipitate, Cu(OH)₂, which does not dissolve in excess NaOH.
Cu²⁺ 产生蓝色沉淀 Cu(OH)₂，该沉淀不溶于过量NaOH。

Fe²⁺ gives a dirty green precipitate, Fe(OH)₂, which slowly turns brown on exposure to air.
Fe²⁺ 产生墨绿色沉淀 Fe(OH)₂，在空气中缓慢变棕色。

Fe³⁺ gives a reddish-brown precipitate, Fe(OH)₃.
Fe³⁺ 产生红棕色沉淀 Fe(OH)₃。

Al³⁺ gives a white precipitate, Al(OH)₃, which dissolves in excess NaOH to form a colourless solution containing [Al(OH)₄]⁻.
Al³⁺ 产生白色沉淀 Al(OH)₃，该沉淀溶于过量NaOH形成无色溶液，其中含 [Al(OH)₄]⁻。

Zn²⁺ gives a white precipitate, Zn(OH)₂, which also dissolves in excess NaOH to form [Zn(OH)₄]²⁻.
Zn²⁺ 产生白色沉淀 Zn(OH)₂，同样溶于过量NaOH形成 [Zn(OH)₄]²⁻。

Ammonium ions, NH₄⁺, do not give a precipitate with NaOH. Instead, warming the mixture releases ammonia gas, which turns damp red litmus paper blue.

铵离子 NH₄⁺ 与NaOH不生成沉淀。加热混合物会释放出氨气，氨气能使湿润的红色石蕊试纸变蓝。

4. Tests for Anions | 阴离子的鉴定

Common AQA tests identify carbonate, sulfate, and halide ions.

AQA常考的测试用于鉴定碳酸根、硫酸根和卤离子。

Carbonate (CO₃²⁻): add dilute nitric acid. Effervescence of carbon dioxide gas occurs, which turns limewater milky.

碳酸根（CO₃²⁻）：加入稀硝酸，产生二氧化碳气体即冒泡，使石灰水变浑浊。

Sulfate (SO₄²⁻): add dilute hydrochloric acid followed by barium chloride solution. A white precipitate of barium sulfate confirms the presence of sulfate.

硫酸根（SO₄²⁻）：加入稀盐酸，再加入氯化钡溶液。产生白色硫酸钡沉淀则证明硫酸根存在。

Halide ions (Cl⁻, Br⁻, I⁻): add dilute nitric acid, then silver nitrate solution.

卤离子（Cl⁻、Br⁻、I⁻）：加入稀硝酸，再加入硝酸银溶液。

Halide ion	Precipitate	中文
Cl⁻	White AgCl	白色氯化银
Br⁻	Cream AgBr	浅黄色溴化银
I⁻	Yellow AgI	黄色碘化银

Add ammonia solution to distinguish the silver halides. AgCl dissolves in dilute ammonia; AgBr dissolves only in concentrated ammonia; AgI is insoluble in ammonia.

可加入氨水区分银的卤化物。AgCl溶于稀氨水；AgBr只溶于浓氨水；AgI不溶于氨水。

5. Tests for Gases | 气体的检验

Identification of gases is frequently examined. You must recall the test and the positive result.

气体鉴定是常考内容。你需要熟记测试方法及阳性结果。

Oxygen (O₂): relights a glowing splint.
氧气（O₂）：使带火星的木条复燃。

Carbon dioxide (CO₂): turns limewater milky.
二氧化碳（CO₂）：使石灰水变浑浊。

Chlorine (Cl₂): bleaches damp litmus paper.
氯气（Cl₂）：使湿润的石蕊试纸褪色。

Ammonia (NH₃): turns damp red litmus paper blue; forms white smoke with HCl.
氨气（NH₃）：使湿润红色石蕊试纸变蓝；遇HCl产生白烟。

Sulfur dioxide (SO₂): turns acidified potassium dichromate from orange to green.
二氧化硫（SO₂）：使酸化的重铬酸钾由橙色变为绿色。

Hydrogen (H₂): gives a squeaky pop when a lighted splint is held to the tube.

氢气（H₂）：用点燃的木条接近试管口时发出“噗”的爆鸣声。

6. Identifying Organic Functional Groups | 有机官能团的鉴定

Organic molecules can be identified by tests that react with specific functional groups. The tables below summarise the key AQA tests.

有机分子可以通过与特定官能团反应的测试进行鉴定。下面的表格总结了AQA的关键测试。

The addition of bromine water is a classic test for unsaturation.

加入溴水是检验不饱和键的经典方法。

Alkenes (C=C) decolourise bromine water rapidly at room temperature.
烯烃（C=C）在室温下迅速使溴水褪色。

Alkanes do not react with bromine water in the dark; in UV light, bromine water may slowly decolourise due to substitution, but the reaction is much slower and produces HBr.
烷烃在暗处不与溴水反应；在紫外光下，溴水可能因取代反应而缓慢褪色，但反应慢得多并放出HBr。

Acidified potassium manganate(VII) is a strong oxidising agent.

酸化的高锰酸钾(VII)是一种强氧化剂。

Alkenes decolourise purple KMnO₄ solution; a brown precipitate of MnO₂ may also form.
烯烃使紫色的KMnO₄溶液褪色；也可能生成MnO₂棕色沉淀。

Primary and secondary alcohols, when warmed, also decolourise KMnO₄, producing carbonyl compounds. Tertiary alcohols do not react under mild conditions.
伯醇和仲醇加热时也能使KMnO₄褪色，生成羰基化合物。叔醇在温和条件下不反应。

7. Tests for Alcohols and Carbonyls | 醇和羰基化合物的测试

Alcohols can be identified by their reaction with sodium metal or by oxidation products, but in the lab a more convenient test uses phosphorus pentachloride.

醇可以通过与金属钠的反应或氧化产物来鉴别，但在实验室中更方便的测试是用五氯化磷。

Adding phosphorus pentachloride (PCl₅) to an alcohol produces white fumes of hydrogen chloride and forms a chloroalkane. The white fumes turn blue litmus red.

向醇中加入五氯化磷（PCl₅）会产生氯化氢白烟并生成氯代烷。白烟能使蓝色石蕊变红。

Carboxylic acids react with carbonates and bicarbonates to release CO₂, which is a simple test to distinguish them from alcohols and carbonyl compounds.

羧酸能与碳酸盐和碳酸氢盐反应放出CO₂，这是将它们与醇和羰基化合物区别开的简单方法。

To test for an aldehyde or ketone, use Tollens’ reagent or Fehling’s solution.

检验醛或酮可用托伦试剂或斐林试剂。

Aldehydes reduce Tollens’ reagent to silver mirror; ketones do not react.
醛能将托伦试剂还原成银镜；酮不反应。

Aldehydes reduce Fehling’s solution to a brick-red precipitate of Cu₂O; ketones do not react.
醛能将斐林试剂还原为砖红色Cu₂O沉淀；酮不反应。

If a substance gives a positive test with 2,4-dinitrophenylhydrazine (Brady’s reagent), it contains a carbonyl group, but it does not distinguish aldehyde from ketone.

如果物质与2,4-二硝基苯肼（Brady试剂）反应呈阳性，说明含羰基，但不能区分醛和酮。

8. Carboxylic Acids and Acid Derivatives | 羧酸和酸衍生物

Carboxylic acids contain the –COOH group. Their aqueous solutions are weakly acidic and turn blue litmus red.

羧酸含有–COOH基团。其水溶液呈弱酸性，能使蓝色石蕊变红。

Add aqueous sodium carbonate to a suspected acid. Effervescence indicates CO₂ and therefore the presence of an acid. Ethanoic acid gives a clear, colourless solution of sodium ethanoate.

向疑似酸中加入碳酸钠水溶液。冒泡说明产生了CO₂，因此存在酸。乙酸生成透明无色的乙酸钠溶液。

Esters can be identified by their sweet, fruity smell and by hydrolysing them to produce a carboxylic acid and an alcohol, which can then be tested separately.

酯可通过其甜味、果香气味识别，并且可通过水解得到羧酸和醇，再分别测试。

Acid chlorides and acid anhydrides react violently with water to produce carboxylic acids and hydrogen chloride gas; this fuming reaction is a positive test for these derivatives.

酰氯和酸酐遇水剧烈反应生成羧酸和氯化氢气体；这种发烟反应是这些衍生物的阳性测试。

9. Spectroscopic Identification | 光谱鉴定

Spectroscopy provides conclusive evidence for an unknown structure. In AQA you need to interpret IR and mass spectra, and for A-level you may also use NMR data.

光谱学能为未知结构提供决定性证据。在AQA中你需要解释红外光谱和质谱，在A-level中可能还会使用核磁共振数据。

Infrared (IR) spectroscopy identifies functional groups by characteristic absorption frequencies. For example, an O–H bond in alcohols shows a broad absorption at 3230–3550 cm⁻¹, while a C=O bond shows a sharp peak at 1630–1780 cm⁻¹.

红外光谱通过特征吸收频率鉴定官能团。例如，醇中O–H键在3230–3550 cm⁻¹处显示宽吸收峰，而C=O键在1630–1780 cm⁻¹处显示尖锐峰。

Mass spectrometry gives the relative molecular mass from the molecular ion peak, and fragmentation patterns help identify structural fragments.

质谱通过分子离子峰给出相对分子质量，碎片峰帮助识别结构片段。

For example, an unknown compound has a molecular ion peak at m/z = 46. Possible structures include ethanol and methoxymethane. A strong IR absorption at 3300 cm⁻¹ confirms an O–H group, so the compound is ethanol.

例如，某未知物的分子离子峰在m/z = 46。可能的结构有乙醇和甲醚。IR在3300 cm⁻¹处有强吸收证实存在O–H基团，因此该化合物是乙醇。

In NMR, the number of proton environments gives the number of distinct signals, and chemical shifts identify the environment. Integration traces give the ratio of hydrogens in each environment.

在核磁共振中，质子环境的数目给出信号峰的数目，化学位移识别环境，积分曲线给出各环境中氢原子的数目比。

10. Worked Example | 实例解析

An unknown organic compound, X, has the molecular formula C₃H₆O₂. It reacts with sodium carbonate to release CO₂. Suggest a structure for X.

某未知有机化合物X的分子式为C₃H₆O₂。它与碳酸钠反应释放CO₂。请推断X的结构。

Since X gives CO₂ with carbonate, it contains a carboxylic acid group, –COOH. The molecular formula C₃H₆O₂ matches propanoic acid, CH₃CH₂COOH.

由于X能与碳酸盐反应放出CO₂，它含有羧基–COOH。分子式C₃H₆O₂符合丙酸CH₃CH₂COOH。

If X did not react with carbonate but decolourised bromine water, it could be an ester with a C=C bond, such as methyl acrylate. Always combine chemical tests with spectroscopic data for a definitive identity.

如果X不与碳酸盐反应但能使溴水褪色，它可能是含有C=C键的酯，如丙烯酸甲酯。要获得确定性结论，通常需要将化学测试与波谱数据结合。

11. Common Pitfalls | 常见错误

Many students lose marks by confusing tests and observations. Here are the most frequent errors.

许多学生因混淆测试和观察结果而失分。以下是最常见的错误。

Writing a chemical equation when the question only asks to identify the product.
题目只要求鉴定产物时，却写化学方程式。

Stating “white precipitate” without naming the substance, e.g. “white precipitate” instead of “barium sulfate”.
只写“白色沉淀”而不写物质名称，如只写“白色沉淀”而不写“硫酸钡”。

Using the wrong reagent, e.g. using NaOH to test for sulfate instead of barium chloride.
用错试剂，例如用NaOH测试硫酸根而不是用氯化钡。

Forgetting to acidify reagents, e.g. adding silver nitrate without nitric acid, allowing carbonate interference.
忘记酸化试剂，例如不加硝酸就加入硝酸银，导致碳酸根干扰。

Confusing aldehyde and ketone by only using Brady’s reagent.
只用Brady试剂，无法区分醛和酮。

Always write the full name, not just a formula, when the command word is “identify”.

当命令词是“identify”时，务必写出完整名称，而不只是化学式。

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Identifying Unknown Substances | 未知物质的鉴定